Catch basin design

[runoff]

To all;

I am seeking some help. I have designed out a small subdivision and placed my catch basins a part based on a past design requirement that I learned. Well, the local authority wants the CBs every 400 ft which seems a little far for my design application. I have been using the Federal standard but this guy seems to be nagging about this one little thing. I get the slopes alright but would like to know what everyone else uses.

Thanks in advance.

 

[msquared48]

Can the CB inlet handle the flows at that spacing? If not, state your case and put them closer together. If not, not to be fecious, just go with the flow.

Mike McCann
McCann Engineering

 

[cvg]

it seems odd that an agency would ask you to space them farther apart than you want. Usually works the other way round... However, I would tend to agree that 400' spacing seems fine to me. Around here they might be spaced even farther apart.

Typically the spacing is tied to the allowable spread of water in the street. Agencies usually specify the maximum allowable "spread" and the spacing of catch basins is up to you to determine based upon the amount of rainfall, cross slope of road and longitudinal slope of roadway which is used to calculate the spread. That said, for subdivision local streets, spread is often allowed to inundate the entire street and catch basins are required to capture the flow before entering the nearest arterial road. Arterials are often designed so that only 1 lane is flooded during the design storm. So spread would be about 12 - 16 feet typically.

 

[runoff]

Oh well, I tend to agree with both of you. I spaced them at 250' with adequate spread with a slope of 2%. I have plenty of fall and the flows seem to work on my models. The gentleman I have been working with is not an engineer and states that to everyone. He is just going off of the city standard for street design. I have tried to tell him that our design works better than the 400' spacing and there will be zero pooling but he does not listen. I have it documented and mssquared I am leaning towards what you said and go with the flow. The owner is now siding with the city because of course, it is going to be cheaper. Thanks for the comments.

RO

 

[PELS]

The 400' spacing requirement you quoted needs some clarification by you. If the street is straight downhill with no low points, then by this requirement a CB is needed 400 feet downhill from the high point and every 400 feet thereafter.

Jurisdictions usually state that length of flow in the gutter must not exceed "___" feet. In this case are you saying 400 feet? I have seen some requirements as high as 600'. If you have low points in the street, you could conceivably have 800 feet between CB's (400 feet on each side of the CB to the high points). This is usually the situation in flat land areas.

I once had a client who judged the storm drain design based on the number of CB's in your system. He always felt engineers were excessive (conservative) in their design, even though the jurisdiction dictates the criteria.

Remember CB's cost money and if you can show the design meets the jurisdiction's design standards and you know the system handles the design storm, you are endeavoring to produce an efficient design for your client. Obviously, public safety is the primary concern, but the jurisdictions' design criteria is based on achiving the public safety. Having too many CB's doesn't do service to your client.

 

[cvg]

Even the most conservative storm drain design will still allow flooding to occur if you can't get the water into the pipe... Catch basins are the key to allowing the flow to enter. Water will take the path of least resistance and flow down the street until it can enter a catch basin.

 

[PELS]

CVG I totally agree, however if you design the grading and storm system correctly, you should have an overland release that provides the runoff a path to flow overland without inundating lots, if the CB's cease to function for whatever reason. We are required to show that overland release path on our plans.

 

[runoff]

PELS and CVG;

Hey thanks for the responses. This is the kind of thought process I like going through to check my reasoning. PELS you are right and I did calculate possible overland releases and conducted a very intense evaluation for each road. My calculations show that the spread on eather side of the road will meet my 250' design based on my cross slope of road and longitudinal slope. With a 400' length, there is a possibility of three places where it overloads my combination inlets and depending on the slope at each station, I could end up with some water in front of someone's driveway. I recalculated that with a spreadsheet I use to verify inlet designs and based it on a 25 year event. My spread is within a 10 foot design, since this subdivision is only a two lane road. I have maintained my slopes at 2.0 to 3.5% but there are the three points I mentioned above where the slope is 1.45 to 1.5% and that is my arguing/worry point. I feel they need to be closer together.

I spoke with some of my colleagues yesterday and we all feel that I keep my design the way it is. It might be conservative but it is better than risking a law suit for our firm. If I have to, I will take it over the permitting group to argue my points because they are making a decision from someone that is not the engineer and is going off of an ordinance that just does not make sense. Thanks for the help. Any other advice would help if you have any.

 

[cvg]

Unless you are working directly for the city, it is the developer who pays for the extra catch basins. The city does not have a right to limit the number. However, street drainage is rarely something to cause lawsuits as flooding in the street usually doesn't cause any damages. I would be more concerned with making sure the pad and finish floor elevations are above the 100-year flood and not worry too much about whether anybody can drive through your subdivision during the peak of a 25-year rainstorm. By the way, 25-year street drainage design is quite conservative to begin with. Around here, most cities use a 10-year storm. One only designs for a 2-year storm.

 

[runoff]

CVG, thanks. You are right I used a 10 year event for my design but wanted to check the 25 year event as an arguing point. I did not make that clear above, sorry. I know the developer pays for the catch basins but this city official is the one making the issue. The developer told me in a meeting yesterday that he will pay the extra costs and trust my judgement. So, in my opinion the article is mute now.